Add markdown preview, add markdown help icon, and adjust styles for markdown output

This PR addresses the following issues: https://github.com/hypothesis/h/issues/826, https://github.com/hypothesis/h/issues/1262, and https://github.com/hypothesis/h/issues/1354

It adds markdown preview and a markdown help icon:
![currentinterface](https://cloud.githubusercontent.com/assets/521978/3974681/694ae362-27f8-11e4-8657-de213bb937ae.png)

I also changed the Placeholder text to add some examples since I wasn't sure whether the icon alone was enough. Need to usertest with users who have never heard of Markdown. 

The PR also improves the markdown output stylings for annotation bodies:
## Before:
### Collapsed:
![beforecollapsed](https://cloud.githubusercontent.com/assets/521978/3974747/b42cdab0-27f9-11e4-8dec-53218c87da08.png)
### Expanded:
![beforeexpanded](https://cloud.githubusercontent.com/assets/521978/3974746/b19ca582-27f9-11e4-84c6-0fb66a60a8d8.png)

## After:
### Collapsed:
![improvedcollapsed](https://cloud.githubusercontent.com/assets/521978/3974686/8e7b0310-27f8-11e4-9ba4-790f15b2ffa6.png)
### Expanded:
![improvedexpanded](https://cloud.githubusercontent.com/assets/521978/3974689/a65cfc86-27f8-11e4-99b7-f02121678495.png)


I don't know what anyone else thinks, but I find this placeholder text to be rather hideous. One nice thing about markdown is that the simple features are activated by plain text practices employed by many people who know nothing of markdown: using asterisks and underscores for emphasis, hyphens and asterisks lists, etc.

It's really not a _problem_ if people don't know there's markdown support, because they'll just write in plain text and not know otherwise.

But of course you're right that user testing might help.